1. Field of the Invention
The present invention relates to a fixing apparatus for fixing a toner image onto a recording material, and an image forming apparatus having the same.
2. Description of the Related Art
As a fixing apparatus for use in an electrophotographic image forming apparatus such as a copying machine and a printer, a fixing apparatus of heat-roller fixing type has been in wide use. The heat-roller fixing-type fixing apparatus includes a pair of rollers, to be more specific, a fixing roller and a pressure roller, that are brought into contact with each other under pressure. By means of a heating section composed for example of a halogen heater, which is placed in each of or one of the pair of rollers interiorly thereof, the pair of rollers are heated to a predetermined fixing temperature. With the pair of rollers kept in a heated state, such as a recording sheet, which is a recording material having formed thereon an unfixed toner image, is fed to a region where the pair of rollers make pressure-contact with each other, namely a so-called fixing nip region. Upon the recording sheet passing through the fixing nip region at certain heating time and a fixing temperature, the toner image is fixed into place under application of heat and pressure.
Moreover, a fixing apparatus for use in a color image forming apparatus generally employs an elastic roller constructed by forming an elastic layer made for example of silicone rubber on a surface layer of the fixing roller. By designing the fixing roller as an elastic roller, it is possible for the surface of the fixing roller to become elastically deformed so as to conform to irregularities of the unfixed toner image, wherefore the fixing roller makes contact with the toner image so as to cover the surface of the toner image. This makes it possible to perform satisfactory thermal fixation on the unfixed color toner image that is larger in toner adherent amount than a monochromatic toner image. Moreover, by virtue of a deflection-releasing effect exerted by the elastic layer in the fixing nip region, it is possible to provide enhanced mold releasability for a color toner that is more susceptible to occurrence of offset than a monochromatic toner. Further, since the fixing nip region is convexly curved in a radially-outward direction so as to define a so-called reverse nip configuration, it is possible to attain higher paper-stripping capability. That is, a paper stripping action can be produced without using a stripping portion such as a stripping pawl (self-stripping action), wherefore image imperfection caused by the provision of the stripping portion can be eliminated.
In such a color image forming apparatus, when the process speed is increased, the heating time is insufficient if the used fixing nip width is kept comparable with the width used during low-speed operation. Accordingly, it is necessary to increase the fixing nip width. One available method of increasing the fixing nip width is to increase the diameter of the fixing roller. Another method is to increase the thickness of the elastic layer to increase the amount of elastic deformation. However, if the diameter of the fixing roller is increased, the curvature of the fixing roller is reduced. Consequently, if only repulsion of the elastic layer is used, the recording material is not easily peeled off from the fixing roller. As a result, the releasing performance of the recording material deteriorates.
Further, increasing the thickness of the elastic layer causes the following problem. In the fixing roller having the elastic layer, the elastic layer in itself exhibits extremely low thermal conductivity. Therefore, an increase in the thickness of the elastic layer may lead, in a case of providing a heating section within the fixing roller, to further reduction in heat transfer efficiency. Furthermore, if the process speed is increased, heat taken away by the recording material which has passed through the nip region can not be compensated because heat transfer from the interior of the fixing roller slows down. Accordingly there is a possibility that the temperature of the fixing roller cannot stay close to a fixing temperature. Thereby, a surface temperature of the fixing roller can not be maintained, and as a result, the fixing failure is generated.
In order to solve such problems, a fixing apparatus of the external heating type which brings into an external heating section with the surface of the fixing roller, is known. Especially, in recent years, a fixing apparatus of the external belt heating type has been proposed which uses an endless belt as an external heating section, which endless belt is heated to a temperature higher than the surface temperature of the fixing roller and is brought into abutment with the surface of the fixing roller.
One fixing apparatus of the above-described external belt heating type is disclosed, for example, in Japanese Unexamined Patent Publication JP-A 2004-198659, where a rotatable fixing roll for fixing an unfixed image formed on a recording material has a belt-like region on its surface, which belt-like region has a length in the direction of the outer periphery of the surface of the fixing roll and extends longitudinally and is heated from the outside by an external heating member.
Further, Japanese Unexamined Patent Publication JP-A 2005-189427 discloses a fixing apparatus having a fixing roller that is rotationally driven by an electric motor. The apparatus further includes support rollers that are rotationally driven by the fixing roller.
In this way, the fixing apparatus of the external belt heating type can directly supply heat to the surface of the fixing roller from which heat is removed by the recording material. This apparatus is adapted for high-speed color fixing applications where the fixing roller has a thickened elastic layer.
In the conventional fixing apparatuses of the external belt heating type as disclosed, for example, in JP-A 2004-198659 and JP-A 2005-189427, the amount of heat supplied to the fixing roller by the external heating means (i.e., the amount of heat flow) is not optimized sufficiently and so there is the problem that the warming-up time is not shortened sufficiently.
Furthermore, depending on the condition of the external belt, contact with a lateral motion-hindering member produces cracks in greatly stressed ends or poor rotation. In consequence, the surface temperature of the fixing roller may not be maintained stably.